In the past few years multilateration has become popular for many aircraft air traffic control applications. Initially introduced for airport surface tracking to prevent runway incursions, the benefits of multilateration have extended to terminal and wide areas. Wide area multilateration (WAMLAT) is viewed as a transition and potential back up to Automatic Dependent Surveillance Broadcast (ADS-B). Since WAMLAT techniques include satellite-based timing, and ADS-B uses satellite navigation, the impact of satellite common mode failures needs to be assessed for combined ADS-B and back up applications. This following is a summary of the availability of timing data from satellite navigation systems and proposes techniques to improve overall availability of WAMLAT.
There are a number of wide Area Multilateration Satellite Synchronization Techniques in the Prior Art. Eurocontrol Report EATMP TRS 131/04, Wide Area Multilateration, Version 1.0, November 2004, by W. H. L. Neven, T. J. Quilter, R. Weedon, and R. A. Hogendoorn, incorporated herein by reference, assessed the various synchronization methods used for WAMLAT. Four methods were evaluated—common clock, reference transponder, and two satellite techniques using standard GNSS processing as well as common view GNSS processing, which is essentially an over-determined solution for timing.
FIG. 1 is a diagram illustrating Standard GNSS Synchronization. Referring to FIG. 1, satellite constellation generates GPS timing signals, which are received by antennas 115, 135, which feed corresponding GNSS receivers 110, 130 at respective multilateration tracking stations. Each multilateration tracking station also has a corresponding down converter 120, 140, for receiving aircraft or other vehicle radio signals. GNSS receivers 110 and 130 feed local clocks 150 and 170 which in turn are used as time sources for time of arrival measurement units 150 and 180, which time stamp the received radio signals from down converters 120, 140, respectively. Through digital links, the time-stamp data is fed to a time difference of arrival (TDOA) and tracking unit 190 where vehicle position can be determined.
FIG. 2 is a diagram illustrating Common View GNSS Synchronization. Referring to FIG. 2, satellite constellation generates GPS timing signals, which are received by antennas 215, 235, which feed corresponding GNSS receivers 210, 230 at respective multilateration tracking stations. Each multilateration tracking station also has a corresponding down converter 220, 240, for receiving aircraft or other vehicle radio signals. GNSS receivers 210 and 230 feed local clocks 250 and 270 which in turn are used as time sources for time of arrival measurement units 250 and 280, which time stamp the received radio signals from down converters 220, 240, respectively. Through digital links, time data is fed from GNSS receivers 210 and 230 directly to processor 295, which corrects timing data, while the time-stamp data is fed to a time difference of arrival (TDOA) and tracking unit 290 where vehicle position can be determined.
There are other satellite based timing techniques that the Eurocontrol study did not evaluate, such as relative timing as presented in U.S. Pat. No. 6,049,304, method and apparatus for improving the accuracy of relative position estimates in a satellite-based navigation system, incorporated herein by reference. The relative timing solution technique results in higher timing accuracy by eliminating errors affecting multiple receivers in the same geographic region. In this approach, the standard absolute navigation equations are modified to solve directly for relative position and timing, thereby providing increased precision.
Wide Area Multilateration has been used to validate ADS-B. While ADS-B promises global accurate tracking of aircraft using a significantly lower-cost surveillance infrastructure than today's conventional radar surveillance, there are issues regarding availability and spoofing. WAMLAT is widely viewed as a potential back up/validation to ADS-B. The Eurocontrol report concluded that WAMLAT could be used in the following roles.
To verify navigation accuracy, ADS-B data can be checked against the multilateration data to verify the track keeping performance of the avionics. ADS-B may also be used for Integrity Monitoring. WAMLAT can be used to monitor the integrity of ADS-B as a surveillance technique. This may be done to gather data for a safety case and to monitor the integrity of in-service systems. For example, a bias in one aircrafts position is a serious safety issue for ADS-B only surveillance but a WAMLAT system could identify this immediately. For Anti-spoofing, WAMLAT systems can be used to identify genuine aircraft and the source of spoof transmissions. However, since both ADS-B and WAMLAT depend on satellite information, the impact of satellite common mode failures should be assessed.
Satellite availability is another issue affecting the use of GPS in wide-area multilateration. In the GPS standard positioning service signal specification, 2nd Edition, dated 1995, and incorporated herein by reference, the minimum coverage availability, which is the probability of four or more satellites in view over any 24-hour interval, averaged over the entire globe is ≧0.999. In a paper titled Weight RAIM for Precision Approach by Per Enge of Stanford University presented at the 1995 ION GPS Conference and incorporated herein by reference, he concluded that P(N≧4) was 0.99996. This result was based on simulation using realistic satellite failure models over 107 simulated geometries.
These results relate to four satellites in view to provide navigation. However, WAMLAT does not need the navigation mode for operation, as the necessary function is timing or relative time measurements between the sensors. Since WAMLAT sensors are stationary, with accurately known positions, solutions using four or fewer satellites are sufficient for time/relative time measurement. In a paper published at the 1999 ION National Technical Meeting, and incorporated herein by reference, Boeing's Clifford Kelley summarized the historical availability of GPS satellites from 1995-1999 as illustrated in Table 1.
TABLE 1Number of OperationalSatellitesAvailability≦21   1.0  220.9992230.9475
Therefore, at any time, there are 21 or more operational satellites making up the constellation from which users need to have four in view for navigation. For the purpose of quantification, the timing availability for AirScene™ using GPS is expected to be significantly better than the requirements for navigation and is concluded to be ≧0.99999. This is considered to be a conservative value and drives the overall system availability.
The United States has implemented a Wide Area Augmentation System (WAAS). An excellent description of WAAS may be found on Mehaffy, Yeazel, and DePriest's GPS information website, http://www.gpsinformation.org/dale/dgps.htm, incorporated herein by reference. WAAS is a method of providing better accuracy from the GPS constellation and it similar in principle to DGPS except that a second receiver is not required. Correction data is sent via geo-stationary satellites (GEO) and is decoded by one of the regular channels already present in the GPS receiver. Thus one of the channels can be designated to decode regular GPS signals or can be used to decode WAAS data. Regional correction data is collected by a set of ground stations all over the United States. The data is packaged together, analyzed, converted to a set of correction data by a master station and then uploaded to the GEOs, which in turn transmit the data down to the local GPS receiver. The GPS receiver then figures out which data is applicable to its current location and applies appropriate corrections to the receiver. Importantly, the GEOs also function as independent GPS satellites and therefore provide another source of timing. FIG. 3 illustrates a WAAS Ground Station Layout.
As of 2006, the WAAS system is operational and there are near-real-time updates on system performance posted on the internet such as the non precision approach coverage from http://www.nstb.tc.faa.gov/npa.html, incorporated herein by reference. For non-precision approach accuracy a DOP of up to four may be used. FIG. 4 illustrates a near real time display of non precision approach accuracy.
The GPS Risk Assessment Report, VS-99-007, January 1999, Johns Hopkins University, incorporated herein by reference, evaluated the improvements in availability provided by various GEO augmentation scenarios. FIGS. 5, 6, and 7 illustrate the use of GPS for various navigation applications, with no GEO augmentation, with GEO augmentation, and with GEO augmentation and assumptions regarding mean time to repair (MTTR). FIG. 5 is a chart illustrating GPS Availability without Augmentation. FIG. 6 is a chart illustrating GPS Availability with Augmentation. FIG. 7 is a chart illustrating GPS Availability with Augmentation and MTTR Assumptions.
Generally, for en-route and terminal navigation applications, navigation availability improves by a factor of 100 or so when four GEOs are used. Navigation availability requirements of 0.99999 are exceeded by at least one order using four GEOs.
Similar wide area correction systems exist in other parts of the world, such as the European EGNS (http://www.esa.int/esaNA/index.html) and the Japanese MTSAT, both of which are incorporated herein by reference. The European ground station network (from http://www.gpsinformation.org/dale/dgps.htm, incorporated herein by reference) is illustrated in FIG. 8. FIG. 8 is a map illustrating European EGNOS Station Locations.
Each correction system, using geostationary satellites, provides higher availability and integrity than un-augmented satellite systems such as GPS or Galileo. FIG. 9 summarizes the overlay provided by each system. Note that the footprints shown are constrained by the location of wide area ground stations, and the GEOs, complete with additional timing information, cover a far broader area. FIG. 9 is a map illustrating WAAS, EGNOS, and MSAS Ground Station Coverage Areas.